The present invention relates to a staged combustion system in which the production of undesirable combustion product components is minimized over the engine operating regime and, more particularly, to a pilot mixer for such staged combustion system having a fuel nozzle with an increased fuel flow and a desired atomization of fuel droplets.
Air pollution concerns worldwide have led to stricter emissions standards both domestically and internationally. Aircraft are governed by both Environmental Protection Agency (EPA) and International Civil Aviation Organization (ICAO) standards. These standards regulate the emission of oxides of nitrogen (NOx), unburned hydrocarbons (HC), and carbon monoxide (CO) from aircraft in the vicinity of airports, where they contribute to urban photochemical smog problems. Such standards are driving the design of gas turbine engine combustors, which also must be able to accommodate the desire for efficient, low cost operation and reduced fuel consumption. In addition, the engine output must be maintained or even increased.
It will be appreciated that engine emissions generally fall into two classes: those formed because of high flame temperatures (NOx) and those formed because of low flame temperatures which do not allow the fuel-air reaction to proceed to completion (HC and CO). Balancing the operation of a combustor to allow efficient thermal operation of the engine, while simultaneously minimizing the production of undesirable combustion products, is difficult to achieve. In that regard, operating at low combustion temperatures to lower the emissions of NOx can also result in incomplete or partially incomplete combustion, which can lead to the production of excessive amounts of HC and CO, as well as lower power output and lower thermal efficiency. High combustion temperature, on the other hand, improves thermal efficiency and lowers the amount of HC and CO, but oftentimes results in a higher output of NOx.
One way of minimizing the emission of undesirable gas turbine engine combustion products has been through staged combustion. In such an arrangement, the combustor is provided with a first stage burner for low speed and low power conditions so the character of the combustion products is more closely controlled. A combination of first and second stage burners is provided for higher power output conditions, which attempts to maintain the combustion products within the emissions limits.
One mixer design that has been utilized is known as a twin annular premixing swirler (TAPS), which is disclosed in the following U.S. Pat. Nos. 6,354,072; 6,363,726; 6,367,262; 6,381,964; 6,389,815; 6,418,726; 6,453,660; 6,484,489; and, 6,865,889. Published U.S. patent application 2002/0178732 also depicts certain embodiments of the TAPS mixer. It will be understood that the TAPS mixer assembly includes a pilot mixer which is supplied with fuel during the entire engine operating cycle and a main mixer which is supplied with fuel only during increased power conditions of the engine operating cycle.
Currently, the pilot mixer of the TAPS mixer assembly has utilized an Airblast-Simplex (“ABS”) type fuel nozzle for providing fuel therein. An exemplary ABS fuel nozzle is disclosed in U.S. Pat. No. 5,224,333 to Bretz et al. It will be appreciated that the ABS injector uses a combination of fuel pressure drop and airblast effect for both pre-filming and atomizing the fuel. The reliance on fuel pressure drop, however, reduces the fuel flow range possible within the limits of pump capacity at high flows. It has been found that the spray stability and atomization quality of the fuel at low flows for the ABS fuel injector is also reduced. These characteristics become even more important as the pressure ratio in gas turbine engines become greater.
Thus, there is a need to provide a mixer assembly for a gas turbine engine where the fuel injectors of the pilot mixer have an increased fuel flow range by improving the fuel flow number. In addition, it is desired that the increase in fuel flow number would not sacrifice spray stability and atomization quality of the injected fuel at low flow conditions. At the same time, any increase in susceptibility to coking for the fuel injectors would be addressed.